The science of illness. Immunology is a branch of biology or biomedical science that covers the study of immune systems in all organisms. The immune system is a defense mechanism of organisms. If the immune system is not functioning as it should, it can result in diseases, such as autoimmunity, allergy, and cancer. Immunology has applications in numerous disciplines of medicine, particularly in the fields of organ transplantation, oncology, virology, bacteriology, parasitology, psychiatry, and dermatology. It deals with the defense mechanisms including all physical, chemical and biological properties of the organism that help it to combat its susceptibility to foreign organisms, material, etc.